{
\def\sym#1{\ifmmode^{#1}\else\(^{#1}\)\fi}
\begin{tabular}{l*{6}{c}}
\hline\hline
            &\multicolumn{1}{c}{(1)}&\multicolumn{1}{c}{(2)}&\multicolumn{1}{c}{(3)}&\multicolumn{1}{c}{(4)}&\multicolumn{1}{c}{(5)}&\multicolumn{1}{c}{(6)}\\
            &\multicolumn{1}{c}{l\_volume}&\multicolumn{1}{c}{l\_volume}&\multicolumn{1}{c}{l\_volume\_adh}&\multicolumn{1}{c}{l\_volume\_adh}&\multicolumn{1}{c}{volume}&\multicolumn{1}{c}{volume}\\
\hline
mode\_cost\_1987&      -4.082\sym{a}&      -3.976\sym{a}&      -5.758\sym{a}&      -3.931\sym{a}&      -8.019\sym{a}&      -7.031\sym{a}\\
            &     (0.156)       &     (0.170)       &     (0.179)       &     (0.169)       &     (0.195)       &     (0.215)       \\
\hline
\(N\)       &        2473       &        1635       &        4356       &        1635       &        4290       &        1635       \\
r2\_a        &       0.543       &       0.522       &       0.551       &       0.521       &                   &                   \\
r2\_p        &                   &                   &                   &                   &       0.764       &       0.729       \\
\hline\hline
\multicolumn{7}{l}{\footnotesize Standard errors in parentheses}\\
\multicolumn{7}{l}{\footnotesize \sym{c} \(p<0.1\), \sym{b} \(p<0.05\), \sym{a} \(p<0.01\)}\\
\end{tabular}
}
